[krobbn]

Think of it this way, if another company announces they're planning to buy a publicly traded company, what happens to the value of the shares?

The price goes up, you've just made it more expensive for yourself to take over the network.

If you were to attempt to take over you'd be better to do so in silence. However, it would be hard to hide that kind of control and wealth when every information on the network is publicly available.

[majormajor]

In a public company, though, having been taken over doesn't defeat the purpose of the company.

If you're intentionally trying to take things down, sellers have a huge incentive to not be left in the 49% who hold something that's now lost its value - as you say, "as it's become evident you've taken over the network the value of the network is essentially worthless." I think you could get the value to go to worthless well before actually hitting 51% on intent alone, if you're a big enough power.